Fuel cells are environmentally clean, quiet, and highly efficient devices for generating electricity and heat from hydrogen, natural gas, methanol, propane, and other hydrocarbon fuels. Fuel cells convert the energy of a fuel directly into energy—electricity and heat—by an electrochemical process, without combustion or moving parts. Advantages include high efficiency and very low release of polluting gases (e.g. NOX) into the atmosphere. Of the various types of fuel cells, the solid oxide fuel cell (SOFC) offers advantages of high efficiency, low materials cost, minimal maintenance, and direct utilization of various hydrocarbon fuels without extensive reforming. SOFC systems operating with natural gas as a fuel can achieve power generation efficiencies in the range of 40 to 45 percent, and even higher efficiencies are possible with hybrid systems. Power is generated in a solid oxide fuel cell by the transport of oxygen ions (from air) through a ceramic electrolyte membrane where hydrogen and carbon monoxide from a hydrocarbon (e.g., natural gas) are consumed to form water and carbon dioxide. The ceramic electrolyte membrane is sandwiched between electrodes where the power-generating electrochemical reactions occur. Oxygen molecules from air are converted to oxygen ions at the air electrode (cathode), and these oxygen ions react with hydrogen and carbon monoxide to form water and carbon dioxide at the fuel electrode (anode).
The same types of materials are used in most of the SOFC systems currently under development. Compositions used for the ceramic electrolyte membrane material include yttrium-stabilized zirconia (YSZ), gadolinium-doped ceria (GDC), and samarium-doped ceria (SDC), among others. The air electrode (cathode) is a ceramic material having compositions such as lanthanum strontium manganite (LSM), lanthanum strontium ferrite (LSF), lanthanum strontium cobalt ferrite (LSCF), samarium strontium cobaltite (SSC), praseodymium strontium manganite (PSM), and praseodymium strontium manganese iron oxide (PSMF), among others. The fuel electrode (anode) is a composite (cermet) mixture of a ceramic electrolyte material (e.g., YSZ, GDC or SDC) and a metal (e.g., nickel or copper). The anode material typically is produced as a mixture of the electrolyte material (e.g., YSZ or GDC) and the oxide of the metal (nickel oxide or copper oxide); prior to operation of the SOFC, the oxide in the composite anode is reduced to the corresponding metal.
Currently, most developmental SOFC systems operate at relatively high temperatures (i.e., 800 to 950° C.). At these high temperatures, the electrode materials provide suitable performance using conventional means of preparation. However, at these high temperatures, with current anode materials, hydrocarbon fuels must first be converted to a mixture of hydrogen and carbon monoxide (for example, by reacting the hydrocarbon with steam); the mixture of hydrogen and carbon monoxide is then delivered to the SOFC where power is generated. Without this external “reforming” step, carbon would deposit onto the anodes of the SOFC and performance would degrade rapidly. Operation of SOFCs at lower temperatures (650 to 750° C.) would allow internal reforming at the anode without carbon deposition, thus reducing size and cost of the system and increasing overall efficiency. Lower operating temperatures also will minimize adverse chemical reactions between component materials, minimize adverse effects of thermal expansion mismatches between component materials, reduce cost by allowing less expensive metals to be used for interconnects and gas manifolds, and reduce the size and weight of the SOFC power generation system by lessening requirements on heat exchangers and thermal insulation.
However, it has been difficult to achieve high SOFC power densities at low temperatures in solid oxide fuel cells because of increased electrolyte resistance and inefficiency of the electrode materials. It has been demonstrated that reducing the thickness of electrolyte membranes lowers electrolyte resistance. This has been achieved in SOFCs with planar geometries by using one of the porous electrodes (typically the anode) as the bulk structural support (about one millimeter thick), depositing a dense thin film (about ten microns) of the electrolyte material on the porous anode substrate, and subsequently depositing the opposite electrode (cathode) as a porous film (about fifty microns) on the electrolyte film surface. Very high SOFC power densities have been achieved at temperatures of 750 to 800° C. with planar SOFCs produced with this type of configuration. However, even better SOFC performance and lower temperature operation will be achieved by using improved electrode (cathode and anode) materials.
Two approaches have been demonstrated for improving low-temperature performance of cathodes in solid oxide fuel cells. The first approach involves replacement of lanthanum strontium manganite (LSM), which conducts electricity solely via electron transport, with mixed-conducting ceramic electrode materials, i.e., materials that conduct electricity via transport of both oxygen ions and electrons. Examples of mixed-conducting electrode materials include (La,Sr)(Mn,Co)O3 (LSMC), (Pr,Sr)MnO3 (PSM), (Pr,Sr)(Mn,Co)O3 (PSMC), (La,Sr)FeO3 (LSF), and (La,Sr)(Co,Fe)O3 (LSCF). The second approach to improving low-temperature cathode performance involves addition of electrolyte material to the electrode material. This improvement is due to increasing the volume of triple-point (air/electrode/electrolyte) regions where electrochemical reactions occur. This enhancement is most effective in LSM when ceria-based electrolytes (SDC or GDC) are added or when the particle size of the component (electrolyte and electrode) materials is reduced. This composite cathode approach also has been shown to provide enhancements for mixed-conducting electrode materials such as LSF and LSCF.
In order to improve anode performance, regardless of operating temperature, it is desired to reduce the respective particle sizes of the metallic and ceramic components of the cermet anode material. The particle size reduction results in an increase in the volume of triple-point (gas/nickel/electrolyte) regions where electrochemical reactions occur. When operation via internal reforming is desired, ceria-based electrolytes may be preferred over YSZ.
Accordingly, there is a need in the art for processes for preparing improved powder mixtures of ceramic electrolyte and electrode materials, and high-performance anode and cathode materials for solid oxide fuel cells prepared using such processes. Specifically, by achieving these powder mixtures on a nanoscale (e.g., less than 100 nm in dimension), improved electrode performance will be obtained. Other applications where advanced electrode materials are needed include ceramic electrochemical gas separation systems, gas sensors, and ceramic membrane reactors.